1. Field of the Invention
The invention relates to a continuous process for the production of vinyl chloride polymers in aqueous suspension.
2. Description of the Prior Art
Vinyl chloride has already been polymerized in an aqueous dispersion on a large industrial scale for many years. Whereas polymerization in an aqueous emulsion in the presence of water-soluble emulsifiers and activators has already been carried out continuously from the start of technical development and, even today, this process is largely used in the art, the discontinuous (batchwise) polymerization process has hitherto been preferred for the polymerization of vinyl chloride in aqueous suspension in the presence of water-soluble suspending agents (protective colloids) and oil-soluble activators. As a result of the continuous increase in the sales of the tonnage plastic polyvinyl chloride and as a result of competition with other tonnage plastics, there has already existed for some years a necessity to produce on an increasingly cheaper basis and an opportunity to produce on an increasingly larger scale. In a trend of this kind the production of polyvinyl chloride in aqueous suspension by a continuous process has advantages, since such a process usually enables better space-time yields to be achieved than a discontinuous process, and the disadvantage of a continuous process, that it is relatively sluggish when changes of grade are made, becomes increasingly less important in view of the increasingly greater prospects for the sales of a single grade.
Although, as a result of industrial application over a period of many years, a whole series of processes and process improvements are known for the continuous emulsion polymerization of vinyl chloride, technical development for suspension polymerization has hitherto proceeded mainly in the direction of continuing batchwise production but in increasingly larger units. The principal reason for this is perhaps that the techniques which are known for continuous emulsion polymerization cannot be applied directly, ie. not on the basis of the considerations which are familiar to the expert, to a corresponding continuous suspension polymerization process.
Among the developments made hitherto in the field of continuous suspension polymerization of vinyl chloride U.S. Pat. No. 3,007,903 discloses a process in which polymerization is carried out in a large number, preferably 5 to 10, of zones arranged in tandem. These zones are represented by a corresponding number of stirred kettles, and the reaction medium overflows from one kettle into the next kettle located below it. The zones can also be sections of a tubular reactor. An inert gas is passed through the various zones, co-current with the polymerization liquor, in order to prevent obstruction of the transitions from one zone into another zone.
German Auslegeschrift No. 1,116,410 discloses a continuous suspension polymerization process for vinyl compounds in which the dispersion of the monomer(s) in the aqueous liquor and the subsequent polymerization are carried out in a number of mechanically stirred conversion zones arranged one above the other. A reaction chamber is used for this purpose, comprising a vertical pressure vessel, the diameter of which is substantially less than its height and which has, located in its vertical axis, a continuous axle carrying three or more impeller wheels which act radially. These impeller wheels are arranged at decreasing distances from one another, passing down the axle, so that the lowest degree of agitation required to comminute the monomer into droplets of the desired size and to maintain a suspension is just achieved. Several vertical baffle plates facing inwards, or similar devices, are advantageously fitted on the inner wall of the reaction vessel, the width of these being about 1/10 to 1/12 of the diameter of the reaction vessel. The baffle plates are offset from one another by an angle of about 90.degree. and serve the purpose of increasing the frictional resistance of the suspension in the reaction vessel.
Furthermore, German Auslegeschrift No. 1,217,069 discloses an apparatus for the continuous polymerization of ethylenically unsaturated polymerizable compounds, also including vinyl chloride, which comprises a reaction space which has rotational symmetry and which is sub-divided into chambers by elements having rotational symmetry, the chambers being connected to one another by small apertures and flow of the reaction material being possible only in the main direction of flow of the polymerization material.
In the simplest case the elements having rotational symmetry are disks, separated from the walls of the reaction space by narrow annular gaps. These disks are fixed on a rotating transmission shaft at intervals which are chosen to suit the requirements of a particular case. An attached drawing shows disk spacings which increase in the direction of flow of the polymerization material, the length/diameter ratio of the chambers separated by the disks being about 0.1 to 0.6. A paddle stirrer is mounted on the rotating axle in the last chamber in the direction of flow. The apparatus is described as particularly suitable for emulsion polymerization, but is stated to be also applicable for polymerization in aqueous suspension.
A further apparatus, in particular for the continuous suspension polymerization of vinyl chloride, is disclosed in German Auslegeschrift No. 2,343,788. It comprises a central, tubular reaction space and a shaft which passes vertically through this space, the reaction space being divided into chambers by separating elements and the chambers being connected with one another by small apertures in the separating elements. These chambers carry ring lines, one of which is in each case connected to a chamber by means of two apertures. Stirring elements can be mounted in all the chambers on the shaft passing through the latter. The elements which separate the chambers in the central reaction space from one another are advantageously fixed to the walls of this reaction space and leave an aperture free in the center through which passes the shaft to which the stirring elements are fastened. It is stated to be advantageous to use 5 to 30 stages (chambers plus ring line). The attached drawing shows an apparatus having 9 chambers, of which 7 have ring lines. An illustrative embodiment describes the polymerization of vinyl chloride in an apparatus corresponding to the drawing and containing no stirring elements on the continuous shaft. The entire apparatus is generally made of stainless steel. In this connection, care is taken that the internal surfaces produced are as smooth as possible, in order not to form starting points for deposition.
As can be seen from the publications mentioned first and last, deposition on the walls constitute a problem in the continuous suspension polymerization of vinyl chloride. In the process first mentioned, attempts are made to keep at least the points most sensitive to obstruction, the connecting pipes, free by blowing gas through them. This method is not suitable for operation substantially without wall deposits in the reaction spaces too, or for avoiding the difficulties caused thereby, such as impairment of heat transfer, contamination of the polymer by coarse particles, and the like. In addition, in the case of polymerization batches which have a tendency to foam as a result of an increased content of surface-active substances, blowing gas through can lead to considerable difficulties. In the second and third of the processes mentioned, the problem of wall deposits is not discussed at all, but they certainly occur in the reactor equipped with baffle plates and in the chambers in the reactor which are sub-divided by the disks and they render a laborious cleaning operation necessary. Similar considerations apply to the apparatus according to the fourth publication described, which is in any case fairly expensive. In this case although it is stated that deposits are prevented by means of smooth internal surfaces and cleaning of the reactor is reported not to be necessary, it is stated subsequently that nevertheless occasionally some wall deposit can be formed, which is removed by having to shut down the whole apparatus and to clean it with the aid of a solvent, for example tetrahydrofuran. Considerable quantities of solvent are required for this purpose in the case of a large industrial plant, which have to be stored separately and recovered, which, as well as the outlay, in itself considerable, required for the comparatively complicated polymerization apparatus, also means, besides, an additional outlay in respect of process technology.
In respect of German Auslegeschrift No. 1,116,410, a comparison, described later in the text, of the polymerization process described by means of the examples in that publication with the process of the present invention shows that, even under favorable circumstances, only an appreciable lower space-time yield can be achieved. Reference has already been made earlier in the text to the formation of a deposit. It has been found that the formation of deposit increases markedly at higher rates of shear. The latter certainly occur in the process of German Auslegeschrift No. 1,116,410.
A continuous process for the preparation of a vinyl chloride polymer in aqueous suspension has also already been suggested, in which the reaction is carried out in at least two reaction zones in which the polymerization mixture is kept sufficiently in agitation for the polymer formed not to be deposited. In the first reaction zone polymerization is carried out to a conversion of 3 to 10% by weight, relative to monomers employed. Plug flow of the polymerization mixture is maintained in all the reaction zones, the polymerization mixture is agitated in the first reaction zone in a manner sufficient to obtain the particle morphology desired, and, particularly in the second reaction zone, polymer deposits are prevented from forming on the surfaces which come into contact with the polymerization mixture. This process offers a number of advantages compared with the processes previously mentioned, for example only slight formation of deposits and improved space-time yield. Although the product obtained thereby already has good properties, there is a constant desire to improve even good properties still further. The present invention is based on the subject of improving further the processes known hitherto for the continuous suspension polymerization of vinyl chloride and the pattern of properties of the products obtained thereby.